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(1) Field of the Invention
This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a method of forming a thermal barrier coating, in which the resulting coating is resistant to infiltration by contaminants present in a high temperature operating environment.
(2) Description of the Related Art
Hot section components of gas turbine engines are often protected by a thermal barrier coating (TBC), which reduces the temperature of the underlying component substrate and thereby prolongs the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Air plasma spraying (APS) has the advantages of relatively low equipment costs and ease of application and masking, while TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure. Similar columnar microstructures can be produced using other atomic and molecular vapor processes.
To be effective, a TBC must strongly adhere to the component and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion (CTE) between ceramic materials and the substrates they protect, which are typically superalloys, though ceramic matrix composite (CMC) materials are also used. An oxidation-resistant bond coat is often employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or a diffusion aluminide coating. During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, these bond coats form a tightly adherent alumina (A12O3) layer or scale that adheres the TBC to the bond coat.
The service life of a TBC system is typically limited by a spallation event brought on by thermal fatigue. In addition to the CTE mismatch between a ceramic TBC and a metallic substrate, spallation can occur as a result of the TBC structure becoming densified with deposits that form on the TBC during gas turbine engine operation. Notable constituents of these deposits include such oxides as calcia, magnesia, alumina and silica, which when present together at elevated temperatures form a compound referred to herein as CMAS. CMAS has a relatively low melting eutectic (about 1190xc2x0 C.) that when molten is able to infiltrate the hotter regions of a TBC, where it resolidifies during cooling. During thermal cycling, the CTE mismatch between CMAS and the TBC promotes spallation, particularly TBC deposited by PVD and APS due to the ability of the molten CMAS to penetrate their columnar and porous grain structures, respectively. This process of CMAS infiltration and resulting stresses during heating and cooling cycles is illustrated in FIG. 1.
Various studies have been performed to find coating materials that are resistant to infiltration by CMAS. Notable examples are U.S. Pat. Nos. 5,660,885, 5,871,820 and 5,914,189 to Hasz et al., which disclose three types of coatings to protect a TBC from CMAS-related damage. The protective coatings are classified as being impermeable, sacrificial or non-wetting to CMAS. Impermeable coatings are defined as inhibiting infiltration of molten CMAS, and include silica, tantala, scandia, alumina, hafnia, zirconia, calcium zirconate, spinels, carbides, nitrides, suicides, and noble metals such as platinum. Sacrificial coatings are said to react with CMAS to increase the melting temperature or the viscosity of CMAS, thereby inhibiting infiltration. Suitable sacrificial coating materials include silica, scandia, alumina, calcium zirconate, spinels, magnesia, calcia and chromia. As its name implies, a non-wetting coating is non-wetting to molten CMAS, with suitable materials including silica, hafnia, zirconia, beryllium oxide, lanthana, carbides, nitrides, suicides, and noble metals such as platinum. According to the Hasz et al. patents, an impermeable coating or a sacrificial coating is deposited directly on the TBC, and may be followed by a layer of impermeable coating (if a sacrificial coating was deposited first), sacrificial coating (if the impermeable coating was deposited first), or non-wetting coating. If used, the non-wetting coating is the outermost coating of the protective coating system. Whether used alone or in combination, the impermeable, sacrificial and non-wetting coatings are deposited as discrete layers on top of the TBC.
While the coating systems disclosed by Hasz et al. are effective in protecting a TBC from damage resulting from CMAS infiltration, further improvements would be desirable. In particular, infiltration of columnar TBC deposited by physical vapor deposition is not directly addressed by Hasz et al., yet is of considerable interest to the aerospace industry.
The present invention generally provides a thermal barrier coating (TBC) system and method for forming the coating system on a component suitable for use in a high-temperature environment, such as the hot section of a gas turbine engine. The invention is particularly directed to a method of forming a columnar TBC system to be resistant to infiltration by CMAS and other potential high-temperature contaminants.
The method of this invention generally entails forming a TBC on the surface of a component so that the TBC has at least an outer portion that is resistant to infiltration by CMAS. The thermal barrier coating is formed by evaporating at least one ceramic source material within a coating chamber of a physical vapor deposition apparatus. A first ceramic composition and a second ceramic composition are then co-deposited by physical vapor deposition so that the entire thermal barrier coating has columnar grains and at least the outer portion of the thermal barrier coating is a mixture of the first and second ceramic compositions. An inner portion of the TBC may be deposited to contain only the first ceramic composition, after which the outer portion of the TBC is deposited to contain the mixture of the first and second ceramic compositions. The outer portion is preferably a continuation of the inner portion, i.e., a change in composition may occur within the TBC between the inner and outer portions, but the TBC is otherwise continuous and not characterized by discrete inner and outer coatings. The second ceramic composition serves to increase the resistance of the outer portion of the thermal barrier coating to infiltration by molten CMAS. More particularly, the second ceramic composition is preferably capable of interacting with molten CMAS, forming a compound with a melting temperature that is significantly higher than CMAS. As a result, the reaction product of CMAS and the second ceramic composition resolidifies before it can fully infiltrate the TBC. The outer portion may also contain at least one platinum-group metal that further inhibits infiltration by molten CMAS. The platinum-group metal maybe co-deposited with the first and second ceramic compositions, or deposited before the thermal barrier coating and then diffused into the outer portion as a result of the parameters employed in the deposition process.
The TBC system of this invention has an increased temperature capability as a result of the TBC having reduced vulnerability to spallation from contamination by CMAS and other potential contaminants. The TBC is not required to be made up of multiple discrete layers as proposed in the past, but instead preferably has a continuous structure with its outer portion enriched with the second ceramic composition and optionally a platinum-group metal. Such a continuous TBC structure can be obtained by first depositing the inner portion to contain the first ceramic composition, and then initiating deposition of the second ceramic composition while continuing to deposit the first ceramic composition, such that the TBC is deposited in a single deposition cycle. According to a preferred aspect of the invention, the same coating cycle is used to deposit the TBC, the platinum-group metal and a metallic bond coat on which the platinum-group metal is deposited, such that the entire coating process is performed in a single operation with a single coating apparatus. In this manner, the additional cost of incorporating the second ceramic composition and the platinum-group metal can be minimized.
Other objects and advantages of this invention will be better appreciated from the following detailed description.